The invention concerns a transmission for a steerable drive wheel of a forklift according to the preamble of the main claim. Such transmissions have been disclosed in DE 31 33 027 or DE 44 24 305. In the forklift the transmissions are pivotable around a vertical axle via a rocking bearing. The transmissions have a spur gear stage and a second reduction stage. The pinion of the spur gear stage can be driven by a slip-on electric motor and drives a spur gear sitting upon a pinion shaft. The pinion teeth of the second reduction stage are located, as a rule, directly upon the vertical pinion shaft. The vertical pinion shaft is supported in the transmission housing by means of roller bearings. The roller bearing in the area of the pinion of the second reduction stage being designated as pinion gearing. The bevel gear of the second reduction stage is non-rotatably connected with the drive wheel by a horizontal output shaft.
The required high reduction ratio of the second reduction stage causes very strong reaction forces on the pinion bearing when the forklift is accelerated or decelerated. The maximum load capacity of the pinion bearing and of the toothing is reached as the load stresses increase. For certain applications, it is not possible to enlarge the dimensions of the transmission, due to a lack of installation space. The outer dimensions are limited by the enveloping circle when the transmission is pivoted and by the housing in the area of the bevel gear. The housing must project below, not above, the rim of the drive wheel in order that damage to the transmission can be eliminated in case of defective tires of the drive wheel.
Departing from the already known transmission, the problem to be solved by the invention is to provide a transmission with a capacity for high load stresses While being limited by preset limits of the outer dimensions and a preset reduction ratio. Particularly, the invention must obtain a higher load capacity at the pinion bearing and at the pinion teeth. The problem is solved by the fact that the second reduction stage is designed as a hypoid wheel set with positive axle offset.
In such a known hypoid wheel set described, for example, in Niemann""s xe2x80x9cMaschinenelementsxe2x80x9d Vol. 3, 1983, the pinion axle does not cut the bevel gear axle. It is offset around the so-called crossing step or hypoid offset. In positive offset, the diameter of the hypoid pinion (with equal bevel gear diameter and equal ratio) becomes larger than in corresponding bevel gear transmissions. The larger pinion diameter makes possible a thicker pinion shaft and therewith a larger design of the pinion bearing. This larger design of the pinion bearing cannot be used in the transmissions known due to the smaller pinion diameter. The positive offset produces a larger helix angle of the pinion, which determines a larger length per tooth that takes part in the meshing and a larger overlap ratio. This has an advantageous effect upon the load capacity.
When the pinion bearing abuts directly axially on the teeth of the hypoid pinion, an advantage is obtained by minimizing the bearing stresses.
The maximum thickness of the pinion shaft is obtained when the diameter of the pinion shaft is equal to the root diameter of the hypoid pinion in the transition between the toothing region and the cylindrical part of the pinion shaft.
A specially silent operation of the transmission and a high load capacity are obtained when the gears of the hypoid wheel set are spiral cut.